1. Field of the Invention
The present invention relates to the papermaking industry and in particular to the recycled paper industry. More specifically, its object is a new method for treating paper pulps using an enzyme solution.
2. Background of the Prior Art
More and more the papermaking industry uses recycled papers. For example, for the manufacture of corrugated cardboard, more and more often raw materials are used which are based on recycled fibers and, at the same time, the number of recyclings is increased. With each recycling, the quality of the raw materials is lessened. To obtain a satisfactory level of mechanical characteristics, refining of the pulps in an aqueous suspension is generally carried out, which leads to difficulties in machinability.
The pulps in aqueous suspension which are ready to be worked on a paper machine can be characterized by various parameters, one of which is particularly significant for predicting the draining capability of the pulp. Thus, the Schopper-Riegler (SR) degree of a pulp is defined as being an element of appreciation of the quality of a pulp for the manufacture of paper. The SR expresses the capability of water to separate from the suspension under conditions defined by standard NFQ 50 003. On a scale going from 0 to 100; a high SR value means a low suspension draining speed whereas a low value means a more rapid draining speed. It has been noted, for example, that a pulp which has undergone a refining operation has an SR which is more or less increased depending on the degree of refining to which it has been subjected, as compared to a pulp which has not undergone or has only slightly undergone such an operation.
This parameter plays a particularly important role in the yield of the paper machine. In effect, in order to increase the yield, the draining phase must take as little time as possible. For suspensions with a high SR, it is necessary to increase the concentration of the pulps in order to maintain the same manufacturing rate. This causes poorer sheet formation since the fibers are more difficult to distribute. It is therefore particularly advantageous to have suspensions with a fairly low SR.
It is considered that when the suspensions have an SR of greater than 25, it becomes desirable to attempt to lower it in order to improve the paper manufacturing conditions. Such a lowering is desirable for two reasons: on the one hand, the yield on the paper machine can be improved due to the acceleration of the draining; on the other hand, the rates of manufacture can be maintained without having to compensate for the slowness of the draining with a lesser dilution of the suspension, and with the risks that such would cause poor sheet formation.
The invention seeks to provide a method for treating papermaking pulps in aqueous suspension, such as suspensions based on recycled fibers, having an SR at least equal to 25 which, through the use of enzymes, enables the SR to be lowered and therefore improves the draining of the suspension and the yield of the method for forming the paper.
In the papermaking industry a certain number of methods are known which call for enzyme preparations.
For example, from French Patent No. FR 2,557,894, a method is known for treating papermaking pulps with an enzyme solution which assists the refining of the pulp, and renders it capable of being transformed into a paper having defined characteristics. In accordance with this method, a particular enzyme solution containing xylanases is reacted on an unrefined pulp which, therefore, has a fairly low SR on the order of 10.
This method seeks, therefore, not to improve the draining of the treated pulp, but to improve its capability of being refined.
From Canadian Pat. No. CA 758,488, a method is also known for refining a papermaking pulp which consists of submitting an unrefined pulp to the action of an enzyme solution which is based in particular on cellulase, pectinol or lipase, and of submitting it simultaneously to mechanical refining. The object sought is also the improvement of the refinement of the treated pulp.
From French Pat. No. FR 2,571,738, a method is also known for treating a papermaking pulp in accordance with which, with a view to providing the pulp with the characteristics of a pulp for chemical usage, an enzyme preparation containing fungic cellulases having a C.sub.1 activity and a C.sub.x activity is introduced to the pulp.